Methods and materials for transferring beat at the interface between a heat-dissipating component, which typically includes various electronic components in semi-conductor devices, to an external heat dissipator or heat sink are well-known in the art. In this regard, the electronic components generate substantial heat which can cause the component to fail catastrophically. Even to the extent the component does not fail, such elevated temperatures can and frequently do affect the component's electrical characteristics and can cause intermittent or permanent changes. Indeed, the life of an electronic component is directly related to its operating temperature, and a temperature rise of so much as 10° C. can reduce the component's life by 50%. On the other hand, a corresponding decrease in 10° C. can increase a component's life by 100%.
According to contemporary methodology, the typical solution to such heat dissipation problems is to provide an external heat dissipater or heat sink coupled to the electronic device. Such heat sink ideally provides a heat-conductive pathway from the heat dissipating component to structures such as fins or other protuberances having sufficient surface area to dissipate the heat into the surrounding air. To facilitate such heat dissipation, a fan is frequently utilized to provide adequate air circulation over the fins or protuberances.
However, essential to any effective system for removing heat from an electronic component to a heat sink requires efficient and uniform heat transfer at the interface between the component and the heat sink. Among the more efficient means by which heat is transferred across the interface between the component and the heat sink has been the use of heat conductive pads. Such heat conductive pads are typically pre-formed to have a shape or footprint compatible with a particular electronic component and/or heat sink, such that a given pad may be easily applied thereto prior to coupling the heat sink to the electronic component.
Exemplary of such contemporary phase change pad-type thermal interface products are THERMSTRATE; ISOSTRATE and POWERSTATE (each registered trademarks of Power Devices, Inc. of Laguna Hills, Calif.). The THERMSTRATE interface comprises thermally conductive, die-cut pads which are placed intermediate the electronic component and the heat sink so as to enhance heat conduction there between. The THERMSTRATE heat pads comprise a durable-type 1100 or 1145 aluminum alloy substrate having a thickness of approximately 0.002 inch (although other aluminum and/or copper foil thickness may be utilized) that is coated on both sides thereof with a proprietary thermal compound, the latter comprising a paraffin base containing additives which enhance thermal conductivity, as well as control its responsiveness to heat and pressure. Such compound advantageously undergoes a selective phase change insofar as the compound is dry at room temperature, yet liquefies just below the operating temperature of the great majority of electronic components, which is typically around 50° C. or higher, so as to assure desired heat conduction. When the electronic component is no longer in use (i.e., is no longer dissipating heat), such thermally conductive compound resolidifies once the same cools to room temperature.
The ISOSTRATE thermal interface is likewise a die-cut mounting pad and utilizes a heat conducting polyamide substrate, namely, KAPTON (a registered trademark of DuPont) type MT. The ISOSTRATE thermal interface likewise is a proprietary paraffin-based thermal compound utilizing additives to enhance thermal conductivity and to control its response to heat and pressure.
The process for forming thermal interfaces according to contemporary methodology is described in more detail in U.S. Pat. No. 4,299,715, issued on Nov. 10, 1981 to Whitfield et al. and entitled a METHODS AND MATERIALS FOR CONDUCTING HEAT FROM ELECTRONIC COMPONENTS AND THE LIKE; U.S. Pat. No. 4,466,483, issued on Aug. 21, 1984 to Whitfield et al. and entitled METHODS AND MEANS FOR CONDUCTING HEAT FROM ELECTRONIC COMPONENTS AND THE LIKE; and U.S. Pat. No. 4,473,113, issued on Sep. 25, 1984 to Whitfield et al., and entitled METHODS AND MATERIALS FOR CONDUCTING HEAT FROM ELECTRONIC COMPONENTS AND THE LIKE, the contents of all three of which are expressly incorporated herein by reference.
As is well-known in the art, by providing a thermally conductive compound that is formulated to have selective phase change properties (i.e., having a melting point such that the compound is solid at room temperature, but liquefies at or below the operating temperature of the electronic component to which it is coupled) advantageously enables the compound to be easily used and handled when applied to the interface between the component and a given heat sink. On the other hand, by assuming a liquid state when exposed to the operating temperature of the electronic component, such thermally conductive composition advantageously is then able to fill the voids created by air gaps at the interface between the electronic component and the heat sink. Once filled, such gaps no longer impose an impediment to efficient heat transfer. As those skilled in the art will appreciate, heat flow across the interface improves substantially with better mechanical contact between the electronic component and the heat sink.
Despite their general effectiveness at transferring heat, however, many thermally conductive compounds currently in use have the drawback of being difficult to package, ship, and apply. In this regard, phase change thermal interface materials tend to be very sensitive and can be easily ablated when handled during manufacturing and shipping processes. Moreover, such compounds are typically difficult to accurately apply into position due to the lack of adhesiveness inherent in such compounds, which in turn causes migration or shifting from the interface surface upon which they are applied. Specifically, due to the generally wax-like nature of such thermal interface materials, such materials are inherently susceptible to deformation and mis-shaping even when subjected to minimal handling or use. As a consequence, such thermal compounds, once ablated or mis-shapen, become substantially compromised as to their ability to transfer heat across an interface. Specifically, such deformation can cause air gaps or voids to form at the thermal interface, which, as a heat conductive medium, is inefficient.
As such, as opposed to being deployed at the time of manufacture, as would be optimal to minimize expense and expedite manufacturing, such materials must be applied at a later time, typically on-site by the end user. Such processes are well-known in the art to not only be labor intensive, but also messy and difficult to handle. The latter factor is exceptionally problematic insofar as the same often results in an excessive loss of product, particularly with respect to thermal grease and other prior art compositions.
To address such shortcomings, attempts have been made to provide thermally-conductive materials formed as free-standing, self-supporting layers which may be formed as sheet-like materials, such as films or tapes, that can be readily interposed between the heat-generating component and heat sink. An example of such a material is disclosed in U.S. Pat. No. 6,054,198, issued on Apr. 25, 2000, to Bunyan et al. entitled “Conformal Thermal Interface Material for Electronic Components.” The interfaces that are the subject of such patent consists essentially of at least one resin or wax component blended with at least one thermally-conductive filler, the latter of which preferably comprises an electrically-nonconductive filler which exhibits a thermal conductivity of about 25 to 50 W/m−° K. In an alternative preferred embodiment, the compound utilizes the combination of a pressure sensitive adhesive and an alpha-olefinic thermal plastic component, along with the electrically-nonconductive filler.
In use, such materials are operative to remain form-stable at normal room temperature in a first phase but are conformable in a second phase to substantially fill the interface between the heat-generating electronic component and heat transfer mechanism. To achieve that end, such compositions are formulated to possess a transition temperature at which the material transitions from the first solid phase to the second molten or glass phase that falls within the operating temperature range of the electronic component.
Despite such advanced formulations, however, such conformal thermal interface materials have the drawback of being formulated to transition from its first solid phase to its second conformable phase at a temperature falling within the operating temperature of the electronic component (i.e., the temperature range in which the device has been designed to operate), and hence requires the generation of substantial heat before the phase change can occur. As such, the superior mechanical contact afforded by the thermally-conductive compound as it assumes its conformal or liquid phase is not attained until substantial heat has already been generated. This is disadvantageous to the extent it compromises prematurely the lifetime of the electronic device.
Another disadvantage of these materials is the chosen thermally-conductive filler which is preferably non-conducting and characteristically exhibits a thermal conductivity of about 25 to 50 W/m-° K. The use of electrically-nonconductive fillers, such as metal oxides, results in the production of a light or off-white material which, in use, becomes substantially more difficult to deploy than materials formulated to have darker shades.
Also exemplary of such thermally-conductive materials include those materials disclosed in U.S. Pat. No. 5,930,893, issued on Aug. 3, 1999, to Eaton entitled “Thermally Conductive Material and Method of Using the Same.” Such material comprises a paraffinic wax, which may further include an ethylene/vinyl acetate copolymer, that is specifically formulated to have a melting temperature above the normal operating temperature of the component to which the same is applied. In application, such thermally conductive compounds are first interposed at the interface between the electronic component and heat sink. Thereafter, the component is operated at a temperature which goes beyond its normal operating temperature such that the thermally conductive compound is caused to melt. While in such liquid state, the thermally conductive compound fills the air gaps and voids present at the interface to ensure a better continuum of physical contact across the interface. The temperature applied to the interface is then reduced such that the thermally conductive compound re-solidifies, with the electronic component subsequently operating at its normal operating temperature. In light of being below the melting point of the thermally conductive compound, such subsequent operation of the component does not cause the thermally conductive compound to change phases. In this respect, once initially applied and heated such that the same liquefies and thereafter re-solidifies, such compounds do not undergo any type of phase change, but rather remain in solid phase indefinitely.
Problematic with the formulation of such materials, however, is that substantial heat must be initially introduced to the interface and must necessarily be above the operating temperature of the electronic component. Such elevated temperatures can thermally damage the electronic component, which ironically is the condition which such materials attempt to avoid. Likewise, because the thermally-conductive materials only assume a liquid phase once during application and thereafter re-solidify, to the extent any air gaps or voids are present following such re-solidification, there will thus permanently be in place an inefficient pathway for heat to flow thereacross.
Accordingly, there is a need for a thermally conductive compound that is easy to handle and apply, effective in filling the voids between and transferring heat away from a given heat-dissipating component to a heat sink and preferably is formulated to assume a selective phase change whereby the compound is in a solid state at room temperature, but liquefies when subjected to higher temperatures just below the temperatures at which electronic devices typically operate. There is further a need in the art for a thermally conductive interface compound that is of simple formulation, easy to produce, possess enhanced adhesive properties to insure accurate placement, and does not require special handling.